In an optical network, dense wavelength division multiplexing (DWDM) permits the multiplexing of multiple optical carriers onto a single optical fiber by using different wavelengths of laser light. In DWDM, each transport channel has only one optical carrier that occupies a fixed optical bandwidth. Since the total usable optical bandwidth of an optical fiber is fixed, a DWDM system may have a fixed number of total optical channels. For example, a typical total usable bandwidth of an optical fiber may be in the range of 5-10 THz, and an associated DWDM system may have a fixed number of optical channels, such as 76 or 128 channels, that share the total usable bandwidth.
In DWDM systems, a nonlinear transmission penalty may arise due to optical nonlinear effects, such as self-phase modulation (SPM) and/or crosstalk between different channels, such as cross phase modulation (XPM). The nonlinear transmission penalty may impact the signal integrity of phase modulated optical signals and contribute to phase noise. Various techniques have been developed to reduce the nonlinear transmission penalty and/or its negative effects. For example, SPM effects may be canceled using nonlinear Schrodinger equation back propagation techniques in calculations after a signal is received coherently so that frequency components within a channel may be corrected for certain frequency and phase changes. However, XPM effects cannot be minimized through back propagation techniques without knowing frequency and phase relationships between the channels. Using a combed light source (i.e., a source that locks together channel frequencies) may reduce frequency uncertainty, but a nonlinear transmission penalty may remain due to phase uncertainty in the channels.